deuter(o)- Sometimes deuto-.
[Greek deuteros, second.]
Examples where the form means ‘second’ include deuteragonist (Greek agōnistēs, actor), the person second in importance to the protagonist in a drama; Deuteronomy (Greek nomos law), the fifth book of the Old Testament, so named because it contains a repetition of the decalogue and the laws given in Exodus; and deuterium, a stable second isotope of hydrogen.
The idea of something secondary appears in deuterocanonical, of sacred books or literary works that form a secondary canon; and deuterostome (Greek stoma, mouth), an organism, such as a starfish, whose mouth develops from a secondary embryonic opening.
Deuto- is now uncommon, appearing for example in deutoplasm, sometimes used for the yolk of an egg.